Japanese Laid-Open Patent Publication No. 2013-113347 describes a rotary actuator that includes an output shaft, an arcuate piston, and a cylinder, which accommodates the output shaft and the piston. A seal is attached to the piston, An arcuate first pressure chamber and an arcuate second pressure chamber are defined in the cylinder. In the rotary actuator, pressure fluid is supplied to one of the first pressure chamber and the second pressure chamber and discharged from the other one of the first pressure chamber and the second pressure chamber to push the piston and rotate the output shaft. The arcuate-piston-type rotary actuator reduces the leakage of the pressure fluid as compared to a vane-type rotary actuator.